1. Field of the Invention
The invention relates in general to semiconductor fabrication technologies, and more particularly to a phase-shifting mask (PSM) for use in photolithography in semiconductor fabrication processes. PSM can eliminate ghost lines that would otherwise occur due to side-lobe effect in the resulted pattern definition.
2. Description of the Related Art
In semiconductor fabrication, photolithography is an important and indispensable technique which is used to transfer circuit layout patterns by means of a mask onto predefined locations on a semiconductor wafer. Many processes in semiconductor fabrications, such as etching and ion plantation, require the use of photolithography. In a photolithography process, resolution and depth of focus (DOF) are two major checkpoints used to appraise the quality of the pattern definition. A high level of integration requires a high resolution of pattern definition since the feature size is very small To increase the resolution, a laser source with a very short wavelength, such as a krypton (Kr) deep ultra-violet laser with a wavelength of 2,480 .ANG. (angstrom), is used as the exposure light in the photolithography process. The use of a short-wavelength exposure light, however, will result in a shallow DOF. To allow high resolution and good DOF, one solution is to use the so-called phase-shifting mask (PSM).
Fundamentally, a PSM is formed by adding phase shifter layers onto a conventional mask in order to cause destructive interference to the light passing through the PSM such that the contrast and resolution of the resulting pattern definition can be increased. One benefit of the PSM is that it can increase the resolution of pattern definition without having to change the wavelength of the exposure light.
FIGS. 1A-1C are schematic, cross-sectional diagrams used to depict three different conventional PSM structures. FIG. 1A shows a typical PSM, which includes a quartz substrate 10, a plurality of chromium (Cr) layers 12 coated over the quartz substrate 10 to serve as the blinding portions of the PSM and a shifter layer 14 that can cause a phase shift to the light passing through it so as to enhance the resolution of the resulting pattern definition from the PSM. FIG. 1B shows a conventional rim PSM, which includes a quartz substrate 10 and a plurality of Cr layers 12 coated over the quartz substrate 10. Each Cr layer 12 is covered by a shifter layer 16 that can cause a phase shift to the light passing through it so as to enhance the resolution of the resulting pattern definition from the PSM. FIG. 1C shows a variation of the rim PSM, which differs from the rim PSM of FIG. 1B only in that here the shifter layers 16 are layered beneath the Cr layers 12.
In the foregoing PSM structures respectively shown in FIGS. 1A-1C, the shifting layer is formed from MoSi.sub.z O.sub.x N.sub.y or SiO.sub.x N.sub.y. One drawback to these conventional PSM structures is that they all require two etching steps, one for defining the Cr layers and the other for defining the shifter layers. The overall fabrication process is therefore quite complex in procedural steps, which makes the manufacturing cost considerably high. Furthermore, the pattern of a shifting layer and the pattern of a Cr layer are designed by hand. Fabrication time is therefore long and mistakes are easily made in the process.